Long Term Evolution (LTE) is an improved universal mobile telecommunication system (UMTS) that provides higher data rate, lower latency and improved system capacity. In LTE systems, an evolved universal terrestrial radio access network includes a plurality of base stations, referred as evolved Node-Bs (eNBs), communicating with a plurality of mobile stations, referred as user equipment (UE). A UE may communicate with a base station or an eNB via the downlink and uplink. The downlink (DL) refers to the communication from the base station to the UE. The uplink (UL) refers to the communication from the UE to the base station.
Despite the improvements in the LTE system, it still faces capacity and efficiency problems with the rapid growth of different mobile users. Specifically, UEs in the mobile network face increasing problems of power efficiency. Power consumption is important for UEs that are powered by battery and for UEs that are using external power supply. The importance increases with the continued growth of device population and more demanding use cases. The rapid uptake of Smartphone subscribers and the launch of different types of mobile devices such as machine type communication (MTC) devices creates additional challenges for power efficiency.
For Machine-to-Machine (M2M) use cases, sensor-like devices with modems run on batteries, the cost of exchange or charge the batteries for a large amount of devices is too high to be accommodated. The battery lifetime may determine the devices lifetime or the network lifetime. From a wide range of applications (e.g., Smartphone Apps or MTC applications), UE battery life also becomes a major concern. A considerable number of applications show traffic patterns that cost unnecessary power consumption, because many of the background applications and background traffic are not optimized for power consumption. Even for scenarios where UEs may consume power from an external power supply, it is still desirable to consume less power for energy efficiency purpose.
Optimization for UE power consumption is thus required with the increased popularity of various mobile applications. The LTE system has introduced discontinuous reception (DRX) in both connected state and idle state. In general, long DRX cycle helps to improve UE battery life and to reduce network signaling overhead. However, long DRX cycle also creates potential issues for LTE systems. UEs in idle state are required to wake up periodically to monitor paging channel (PCH) for downlink data. For some services or devices, current paging cycle is too short and therefore not power optimized. The current maximum DRX cycle for a LTE device is 2.56 second, i.e. a UE wakes up for one millisecond every 2.56 second. If the current way of calculating paging occasion does not change, then the paging cycle extension is limited by System Frame Number (SFN) wrap around, which is 10.24 second. Longer than that, the current way of calculating paging occasion does not work.
Another issue is that a very long sleep cycle may affect UE mobility. When a UE is in long sleep cycle, it does not perform mobility measurements. The UE only performs measurements for mobility evaluation at each wakeup. The network thus ends up with less accurate measurement to make assistance efficiently in preparing handover for the UE. Furthermore, with a DRX cycle in the magnitude of minutes, it is likely that cell reselection does not work and cannot be used. This is because the UE is likely to have moved a long distance and away from the coverage of the originally camped cell. When the UE wakes up for paging, the stored cell reselection parameters for the previous cell are no longer applicable. As a result, the UE has to do cell selection. To implement network policy and limit the effort of full scanning, it is desirable for the network to still be able to provide cell reselection parameters that are valid in a wider area for these UEs with long sleep cycle.
Another potential issue of long sleep cycle is paging robustness, because paging occasions are non-synchronized between cells. This means that when a UE wakes up and is out of coverage of its originally camped cell, the UE needs to camp on a new cell to receive paging there, and wait (in sleep mode) for paging there. For a faster moving UE that changes coverage cells many times during sleep, the result may be that the wakeup time for paging is always wrong, i.e. the UE calculates its paging occasion based on the parameters of the old camped cell for which the UE is no longer in the coverage when it wakes up. Even for a UE that does not change cell very often, very long sleep also results in clock drift and risks of missing pages due to monitoring paging at wrong timing. In addition, if the paging cycle is very long, a longer wakeup session is expected since the UE may need to go through a longer preparation. For example, timing drift may be up to one second for long paging cycle, thus one millisecond wakeup time is not sufficient for UE to prepare for synchronization. In general, if paging cycle is extended up to ˜10 second, it seems feasible to just extend the current paging mechanism. However, if paging cycle is extended to the magnitude of minutes or more, then it seems that the impact is much bigger. A new paging mechanism is desired to improve the robustness and flexibility of paging.